A Reanalysis of ¹⁴N Nuclear Quadrupole Coupling and Methyl Internal Rotation in the Rotational Spectra of Monomethyl Oxazoles and Isoxazoles

Abstract

Previously published rotational frequencies of 2-, 4-, and 5-methyl oxazole and of 3-and 5-methyl isoxazole are reanalysed to obtain correct rotational constants (A constants in the first place) on the basis of hypothetically unsplit rotational centre frequencies. With these, r 0 -structural parameters of the respective methyl group could be determined, thus allowing us to consider the influence of methyl substitution on the N-quadrupole coupling tensor and the structural relevance of the internal rotor angle given by torsional analysis.14 N nuclear quadrupole hfs and IAM methyl torsional analyses were redone with the correct rotational constants.

The microwave spectrum of 2-methylthiazole: 14N nuclear quadrupole coupling and methyl internal rotation

“…The methyl torsional barrier of 34.1 cm −1 in 2MTA is much lower than that found in the oxygen analog 2-methyloxazole (252 cm −1 ). 18,30 This is in agreement with the trend observed for the monomethyl derivatives of furan and thiophene, e.g., 2-methylfuran (416.2 cm −1 ) 31 vs 2-methylthiophene (194.1 cm −1 ) 29 and 3-methylfuran (380.5 cm −1 ) 32 vs 3-methylthiophene (258.8 cm −1 ). 33 The same is also found for the dimethyl substituted versions with the only example 2,5-dimethylfuran (439.2 cm −1 ) 34 vs 2,5-dimethylthiophene (248.0 cm −1 ).…”

The LAM of the Rings: Large Amplitude Motions in Aromatic Molecules Studied by Microwave Spectroscopy

“…An unexpectedly low torsional barrier of only 34 cm −1 is found for the methyl group in 2-methylthiazole (3) [98,99], while similar values in the intermediate range are observed for the 4-( 4) and 5-isomers (5) [100,101]. The value remains low in 2-methylimidazole (21) (123 cm −1 ) [114] and doubles to 252 cm −1 in 2-methyloxazole (11) [107], but is still the lowest value of the methyloxazole class compared to 428 cm −1 in 4-methyloxazole (12) [107,108] and 478 cm −1 in 5-methyloxazole (13) [107]. We notice the remarkably close barriers between the two pairs 4-methylisoxazole (15) (258.4 cm −1 ) [110] and methylfurazan (24) (252.5 cm −1 ) [115] as well as N-methylimidazole (20) (185.1 cm −1 ) [114] and N-methylpyrazole (25) (185.4 cm −1 ) [116].…”

“…Lacking only the study on 3-methylisothiazole, the collection is almost complete. We see a very clear trend that the barrier hindering the methyl torsion is always higher in the oxygen analogues than in the sulfur analogues, i.e., 2-methylthiophene (1) (194 cm −1 ) [96] vs. 2methylfuran (9) (413 cm −1 ) [49,105], 4-methylthiazole (4) (357 cm −1 ) [100] vs. 4-methyloxazole (12) (428 cm −1 ) [107,108], or 5-methylisothiazole (8) (63 cm −1 ) [104] vs. 5-methylisoxazole (16) (272 cm −1 ) [109]. Note that all given values are experimentally determined.…”

“…If the methyl group is located in close proximity of an atom or a group of atoms, then the barrier to methyl internal rotation increases to about 400-600 cm −1 due steric hindrance, as observed in o-methylanisole [127], 2-halogenotoluene [128], 2-fluoro-4-chlorotoluene [129], 2-chloro-4-fluorotoluene [130], 1,2-dimethylnapthalene [131], 3,4-dimethylfluorobenzene [79], or 3,4-dimethylbenzaldehyde [81]. Predicting whether steric effects occur seems evident, but sometimes the estimation of values for methyl torsional barriers fails because of electronic effects, as in the cases of the 2-isomers of methylthiazole (3) [98,99], methyloxazole (11) [107], and methylimidazole (21) [114] mentioned above. Those methyl groups are sterically more hindered than those in the 4-and the 5-isomers, because an oxygen, a nitrogen, or a sulfur atom is larger than a carbon atom, such that a lower barrier cannot be explained by steric effects, especially in 2-methylthiazole (3) with its extremely low barrier of 34 cm −1 [98,99].…”

Two equivalent methyl internal rotations in 2,5-dimethylthiophene investigated by microwave spectroscopy

“…Monomethyl derivatives of unsaturated heterocyclic five-membered rings have been widely studied by microwave spectroscopy. The barriers to internal rotation of the methyl group cover a wide range from low barriers such as in 2-methylthiazole (1), 1 N-methylpyrrole (2), 2 and 4-methylisothiazole (3) 3 to intermediate barriers like in 4-methyloxazole (4), 4 2-methylfuran (5), 5 and 3-methylfuran (6), 6 as well as 2-methylthiophene (7) 7 and 3-methylthiophene (8), 8 as summarized in Fig. 1.…”